It is known in the art to produce thermoplastic resin particles imbibed with a blowing agent for expansion at a later time. In the case of polystyrene foam products, generally a blowing agent such as pentane is impregnated under elevated pressure and temperature in polystyrene. The impregnated particles are then heated, usually in steam or air and the particles foam. The foamed particles can then be placed in a mold and heated again to the point where the particles fuse. The result is a quite rigid low density product which has many uses from packaging the insulating board.
There are many disclosures in the art for imbibing polystyrene particles. Early processes are summarized in Frisch & Saunders, "Plastic Foams", Marcel Delker, Inc., N.Y., 1973, Part II, pp. 525-544, such as diffusion of blowing agents into polystyrene, polymerization of styrene solutions of volatile hydrocarbons, suspension polymerization systems, deposition of expandable polystyrene from solution, quenched-pellet processes and water-in-monomer polymerizations.
There have been a variety of attempts to produce expandable pellets by incorporating the blowing agent in a polymer melt in an extruder and cutting the extrudate into pellets. Collins, in U.S. Pat. No. 3,250,834 extrudes strands of polystyrene containing pentane into grooves onto rotating rolls maintained at a temperature below the expanding temperature of the extruded material. Other disclosures employ underwater pelletizers, see e.g. Biglione et al, U.S. Pat. No. 4,606,873; Gwinn, U.S. Pat. No. 4,385,016; Muirhead et al, United Kingdom Patent No. 1,062,307; Suh, U.S. Pat. No. 4,386,165; and Allen et al in commonly assigned U. S. Pat. application, Ser. No. 093,317, filed Sep. 4, 1981. Hambrecht et al in German Patent DE 3,220,856 discloses melting a polyphenylene ether/polystyrene blend and mixing with a blowing agent at a pressure of 25 to 250 bars, but does not describe making expandable pellets.
It is also known to incorporate a blowing agent into the thermoplastic resin during polymerization. Kajimura et al, U.S. Pat. No. 4,442,232 teaches the addition in the blowing agent to the reaction system in the production of expandable styrene-maleic anhydride copolymers. See also Hann et al, U.S. Pat. No. 4,459,373.
At present the most common method is to impregnate the thermoplastic particles with blowing agent in a suspension. Kajimura et al, U.S. Pat. No. 4,303,756 describes impregnating thermoplastic resin beads with blowing agent in an aqueous suspension under pressure and at temperatures of about 80.degree. C. Krutchen et al, in U.S. Pat. No. 4,532,263 discloses imbibing polyetherimide resins in an excess of a solvent selected from the group consisting of methylene chloride, chloroform and 1,1,2-trichloroethane at temperatures up to about 100.degree. F. DiGiulio, U. S. Pat. No. 4,661,386 discloses carrying out the impregnation of pentane in polystyrene in a slurry of water at 90.degree. C. stabilized by finely divided calcium phosphate and an anionic surfactant. Allen et al, in commonly assigned U.S. Pat. No. 4,782,098 discloses suspending the thermoplastic resin beads in water containing a suspending agent in an autoclave, heating the suspension and introducing the blowing agent under pressure to produce the expandable thermoplastic beads.
These emulsion imbibing processes are highly capital and labor intensive because they are conducted at high pressures and elevated temperatures. It would be a notable advance in the state of the art to discover a much simpler but equally effective imbibing process. Surprisingly, it has now been found that by adding the desired amount of blowing agent to the thermoplastic resin beads, agitating and then storing the mixture in a closed container at ambient conditions for a sufficient amount of time there is produced expandable thermoplastic beads. One can imagine a continuous process in which the blowing agent and thermoplastic resin beads are mixed in a container, such as a polyethylene lined bag, and after sitting for a period of time, opening the bag and having expandable beads ready for use.